Most of the ferrite materials used in a variety of those electronic devices and precision instruments which utilize magnetic properties have been manganese zinc ferrite and nickel zinc ferrite.
Although available in the form of a large single crystal with relative easiness, manganese zinc ferrite is poor in corrosion resistance and when, for example, subjected to a chemical etching process using a photomask, abnormally etched holes tend to occur locally so that various difficulties are encountered in its actual use.
A large single crystal of nickel zinc ferrite, on the other hand, though much superior in corrosion resistance, is difficult to produce and there has been no established process for the commercial production of high-quality nickel zinc ferrite single crystals. It has been produced, heretofore, for the most part by sintering or hot-pressing of powdered materials. The resulting product, however, often contains many internal defects such as void and has suffered from detached crystal grains during cutting and polishing or processing so that a uniform magnetic substance with smooth surface has hardly been obtained. Electrolytic plating can produce a ferrite film which, however, is not necessarily of desired quality, because of well-known difficulties in controlling composition of a ternary plating system.
There is also a process in actual use, which produces ferrite film by chemical vapor deposition (CVD) on a single crystal substrate. This process includes a difficult regulation of composition of the growing ferrite film. When nickel zinc ferrite film is grown, the reaction system comprises three vaporizing parts of reactants and a growing part, all of which require precise temperature control in addition to flow rate regulation of at least one carrier gas. Moreover, growth rate of the ferrite film is low in CVD process, requiring several hours to obtain a film of 20 .mu.m thickness and resulting in operational inefficiency which makes it difficult to supply a large quantity of the ferrite film.
U.S. Pat. No. 2,848,310 discloses a method of making nickel zinc ferrite, which comprises dissolving the ferrite constituents in a lead oxide melt at high temperatures, introducing seed crystals, and growing a single crystal ferrite on the seed crystals by gradually reducing the temperature of melt to deposit the ferrite. It is impossible, therefore, to form a ferrite of special composition or of a large size or to grow continuously ferrite single crystals.
Japanese Patent Application Laid-Open No. 10,975/72 [U.S. Pat. application Ser. No. 89,632 (Nov. 16, 1972]discloses a liquid phase epitaxial growth method for garnet as a means to form a large size of single crystal film. One of the important problems in the liquid-phase epitaxial growth method is the choice of a substrate conforming to the conditions of deposition and to the properties of the deposit. Another problem is to select the melt composition to maintain stable conditions for deposition. These selections vary in a wide range depending on the intended product to be epitaxially grown and must be properly done in each individual case.